A common type of electronic ballast employs a boost converter in combination with a resonant inverter that supplies high frequency current for efficiently powering one or more fluorescent lamps. Use of a boost converter provides a number of important benefits, including a high degree of power factor correction, low harmonic distortion in the AC line current, and load regulation.
U.S. Pat. No. 5,144,195 discloses an electronic ballast that uses the startup delay of the boost converter (i.e., the period of time between application of AC power to the ballast and startup of the boost converter) to provide a period during, which the lamp filaments are preheated prior to ignition of the lamps. An asymptotic plot of the boost converter output voltage for such a ballast is shown in FIG. 1. The ballast disclosed in U.S. Pat. No. 5,144,195 economically provides filament preheating in a "passive" manner and thus avoids the need for dedicated filament preheating circuitry.
For many such electronic ballasts, and particularly those that are designed for powering more than two lamps, it is highly desirable, if not essential, that operating losses in the inverter be minimized in order to enhance ballast efficiency and to reduce the ballast operating temperature so that long-term reliability of electrical components is ensured. Toward this goal, it is highly advantageous to design the ballast with an elevated boost output voltage. Since, for a given lamp load, a higher boost output voltage results in a proportionately smaller flow of current through many electrical components of the inverter, inverter power losses can be reduced considerably if a higher boost output voltage is used.
Unfortunately, although operating with an elevated boost output voltage helps reduce inverter power losses and enhance ballast energy efficiency, it also tends to interfere with the desirable function of using the boost startup delay period to passively provide a filament preheating period. As a simple rule, when the ratio of the boost operating voltage to the peak value of the AC line voltage exceeds a certain value, it becomes increasingly difficult to passively provide adequate filament preheating without violating other critical design constraints. This problem is particularly acute in ballasts that are designed to operate multiple lamps which are connected in series with each other.
The filament preheating voltage that is provided by a ballast like that disclosed in U.S. Pat. No. 5,144,195 may be computed by the following equation: ##EQU1##
As a quantitative example that illustrates the nature of the problem, consider a ballast for powering four series connected T8 type fluorescent lamps from a 120 volt (rms) AC source. In order to optimize boost converter efficiency and power factor correction, the boost output voltage, V.sub.BOOST,OPERATING, for a ballast that is powered from a 120 volt (rms) AC source is typically set in the range of about 250 to 275 volts. Let us assume here that a boost output voltage of 250 volts is desired. V.sub.BOOST,PREHEAT, which is the output voltage of the boost converter prior to startup, is approximately equal to the peak value of the AC line voltage, which is equal to 120 * 1.414, or 170 volts. Let us assume that it is known that the ballast output voltage, V.sub.OUT,IGNITION, must be about 800 volts (rms) in order to ignite the lamps following proper preheating of the filaments, and that the output voltage after the lamps are ignited and operating, V.sub.OUT,OPERATING, is about 600 volts (rms). Finally, it is specified that V.sub.FIL,OPERATING, which is the filament voltage provided to the lamps under normal operation, should be no greater than 4.0 volts (rms) Substituting these values into the above equation gives: ##EQU2##
The above value for V.sub.FIL,PREHEAT, if applied for approximately 500 milliseconds, is generally considered sufficient for preheating of the filaments prior to igniting the lamps.
Let us now assume that a higher boost operating voltage, say 400 volts, is needed in order to reduce operating losses in the inverter. In this case, if we rely on the boost startup delay period to provide filament preheating, we obtain: ##EQU3## which is insufficient for preheating of the lamp filaments.
From the preceding example, it should be clear that increasing the boost operating voltage past a certain point tends to preclude proper filament preheating in ballasts that use what might be termed the "passive approach". One solution to this problem that is well-known in the prior art is to abandon the passive approach entirely and instead use dedicated filament preheating circuitry. Unfortunately, such dedicated circuitry is quite extensive and may add dramatically to the material cost and physical size of the ballast.
It is thus apparent that a method and circuit for ensuring adequate filament preheating in a ballast with an elevated boost voltage that does not require extensive or costly additional circuitry would constitute a significant improvement over the prior art.